The relationship between ocean surface wind and current derivatives as observed by high-resolution Doppler scatterometry

Ernesto Rodriguez, Jet Propulsion Laboratory, Pasadena, CA, United States and Alexander Grant Wineteer, Jet Propulsion Laboratory/California Institute of Technology, Pasadena, CA, United States
The transfer of momentum from ocean surfaces winds occurs relative to the moving ocean surface, so that one expects surface currents to modulate surface wind stress. A consequence of this mechanism, is that wind stress curl will be negatively correlated to surface current relative vorticity, and so will act to remove energy from small scale circulation. Large-scale spaceborne observations have shown that a wind-speed dependent quasi-linear relation exists between these two quantities and can dominate wind stress curl variability. At submesoscales, validation of this quasi-linear relation has been observed in high-resolution coupled models, but, due to the difficulty in obtaining coincident measurements of wind stress curl and relative vorticity from in situ data, the details of the modulation have not been validated. Due to the presence of frontal current features at submesoscales, one expects the surface current modulation to become more important in terms of atmospheric feedback on ocean circulation in this regime, and validation of this interaction is an important topic in air-sea interaction.

Recent advances in radar remote sensing of surface currents and winds using Doppler scatterometry offer the possibility for making simultaneous observations of surface neutral winds (related to stress) and surface currents. In particular, the NASA/JPL DopplerScatt system is able to measure simultaneous surface winds and currents at kilometer scales, and thus provides an invaluable tool for examining these interactions. Here, we report on observations of the coupling between wind stress curl and surface current relative vorticity obtained over a significant range of ocean wind speeds and current conditions obtained over multiple years of DopplerScatt operations. We also demonstrate that, at these scales, surface current divergence shows a relationship with wind divergence, albeit at a lower correlation level.